I have still one light blue key in my inventory in WA.

[mikamocha_ye]

I mean this key (see picture below)

What for is that? I have collected 105 stars (no purple shard because I cheated the level "purple shard".

[yot yot5]

Was it collected in "Frozen Doors" (I think it's called that). It's an alternate solution where you can save a key. Not very useful, but it's fun to keep it.

[mikamocha_ye]

But for what use? I use cheats on 2/3 of the levels, because the game is too hard for me.

[Muzozavr]

In "Frozen Doors" you can get a YELLOW key, not a blue one.

The extra is not required, but you can use it to open up some other door later in the game. The yellow key from "Frozen Doors" can later be used to skip half of "One-Way Tickets", for example.